In recent years, a technique has been actively developed which decreases an engine displacement (downsizes an engine) and ensures a power performance of a supercharger such as a turbocharger in order to improve a fuel efficiency of a vehicle and reduce a carbon dioxide discharge amount.
Since the turbocharger used when downsizing an engine is driven by exhaust energy and a rotation body of the turbocharger has an inertia, it is characterized in that it is difficult to obtain a supercharging pressure (boost pressure) in a transient driving state with respect to a normal driving state.
As illustrated in FIG. 3, even when the engine displacement is decreased by downsizing the engine, it is possible to improve the engine torque and realize the power performance equal to the large-displacement engine by equipping with the turbocharger. However, torque-up may not be expected in an operation area when starting at the sloping road, that is, an area (an area surrounded by the dashed line in FIG. 3) with a high load and a low engine speed (engine rpm). This is caused by the following reasons. When starting the vehicle, the engine speed is extremely low, the exhaust energy (exhaust gas flow rate) is small, and the power for rotating the compressor may not be obtained. Again, since the rotation body of the turbocharger does not have inertia at the same transient condition like the starting operation, the turbocharger may not be easily driven and the supercharging operation may not be substantially performed. Accordingly, there are many limitations in the hill start performance of the vehicle when downsizing the engine.
Furthermore, the information on the related art according to the invention of the application is disclosed in Patent Documents 1 and 2 and Non-Patent Document 1.